Papers by Keyword: Bainite

Abstract: The pipeline steel as an application in pipeline construction must have good comprehensive mechanical properties due to the harsh environment of the pipeline engineering. So this experiment takes the X80 pipeline steel as the research object, the thermal stability second phase particles which would not be dissolved or aggregated at high temperature will be expected by means of adding nanomagnesium oxide into the steel with the method of carrier dispersion addition. The effect of nanometer magnesium oxide addition on the cast microstructure of X80 pipeline steel was analysed. The results show that the cast microstructure is consist of the ferrite and a small amount bainite. And the bainite is distributed at the boundary of the ferrite grains. When adding 0.02 wt% nanometer magnesium oxides, the number of bainite increases significantly in the cast microstructure, which is mostly distributed at the boundary of the ferrite grains.

Abstract: The effects of cooling rate and austenite structure on bainite formation was investigated by means of electron backscatter diffraction analysis and processing of obtained orientation data. Variant pairing tendency of bainitic ferrite was found to depend on the austenite grain size, austenite plastic deformation and cooling rate. In the bainite formed at low cooling rate the variant pairs having the same Bain axis correspondence are more frequent, while at high cooling rate the variant pairs having the same parallel correspondence of close-packed planes are formed side by side preferably. At the same time, these features are influenced significantly by structural state of parent austenite.

Abstract: In this study, medium carbon low alloy steel was used to obtain bainitic structures. The lower bainite and tempered martensite-lower bainite structures were achieved by isothermal austempering and up quenching treatment, respectively. Based on the results obtained these structures showed a very good combination of strength and toughness. Furthermore, it has been shown that austenitization time and temperature, as well as austempering time and temperature play a major role in achieving ultra-high strength bainitic steels.

Abstract: In this work, the bainite transformation during isothermality of aluminium bearing TRIP steel was studied by dilatometric experiment on a pushrod Formastor-F highspeed dilatometer with radio frequency induction heating. The aluminium bearing TRIP steel consisted of the microstructure of austenite, ferrite and bainite in the form of laths which produced by intercritical annealing and isothermal transformation followed by rapid cooling. The effect of intercritical annealing temperature and isothermal transformation temperature were studied by dilatometric experiment and JEOL JXA-8530F Electron Probe Microscopic Analyzer (EPMA).

Abstract: This paper deals with the development of submerged arc welding wires for longitudinal double submerged arc welded (LDSAW) line pipe weld through transmission electron microscopy (TEM). The experimental procedure in the paper describes the test coupons preparation for submerged arc welding (SAW) with various combinations of wire and flux with varying level of alloying element. Microstructural characterization by transmission electron microscopy has been carried out to establish the desired microstructure in the weld of LDSAW for manufacturing the API-5L X-120M line pipes. The TEM micrographs for shows the lath type ferrite and bainitic type ferrite with high dislocation density. The lath type ferrite and bainitic type ferrite with high dislocation density also have fine precipitates in the ferrite matrix having orientation relationship. The weld metal suitable for X-120M have the microstructure of mainly bainitic and martensitic with high dislocation density and coarse precipitates in the matrix. The bainitic and martensitic microstructure have excellent fracture toughness down to -20 °C at this strength level (X-120M).

Abstract: There has been proposed an innovative thermal treatment of bearing steel 100CrMnSi6-4, where the existing standard heat treatment has been replaced by austempering. The structure of low-temperature tempered martensite has been replaced by a microstructure composed of martensite and lower bainite with midrib. The kinetics of bainitic transformation and isothermal martensitic transition at selected austempering temperatures was controlled by acoustic emission. The research on contact strength was made under the conditions of rolling-sliding friction. The microstructure was revealed with the use of a light microscope and the forms of pitting wear were displayed by a scanning electron microscope. It was found that the optimum microstructure providing the best used contact strength of the tested steel is conditioned by the formation of a lower bainite with midrib at the temperatures near MS. A plausible cause of the increased resistance to pitting is bifurcation of fatigue cracks on dispersion bainitic carbides in combination with primary carbides, in bainitic-martensitic matrix.

Abstract: Welding thermal cycle simulation with the heat input of 12~25 kJ/cm and practical welding were undertaken to investigate the weldability of a Zr-Ti microalloyed pipeline steel X120. The microstructure in the simulated coarse-grained heat-affected zone was predominantly bainite. The Vickers hardness and room temperature impact toughness of simulated coarse-grained heat-affected zone is 276~297 (HV10) and 208~225 J, respectively. These results indicated that the X120 steel had good weldability. Practical plate welding with the heat input of 21 kJ/cm also showed that the Zr-Ti microalloyed pipeline steel X120 had high yeild strength (895 MPa) and low temperature (-30°C) impact toughness (183 J, 204 J and 208 J in the fusion line, coarse-grained heat-affected zone and fine-grained heat-affected zone, respectively).

Abstract: Abstract:The microstructures and mechanical properties of 22Mn2SiVBS steel after austenitizing at 1280°C and then cooling in air and piled up after rolled were studied in this paper .the results show that when 22Mn2SiVBS steel cooling in air it can obtain the fine grain granular bainte and remain austenite,and piled up after rolled it can obtain granular bainte and proeutectoid ferrite. After heat treatment the strength and hardness is higher than the sample piled up after rolled, but the toughness is lower.

Abstract: In the present paper, controlled cooling in different ways was performed using a laboratory hot rolling mill in ultra-high strength hot rolled ferrite-bainite dual phase (DP) steel. The results have shown that the final microstructures of DP steel comprise ferrite, bainite and a small amount of retained austenite and martensite. DP steel has a tensile strength ranging from 1010 to 1130MPa and yet retains considerable total elongation in the range of 14–17%. The addition of Mn and Nb to DP steel leads to the maximum ultimate tensile strength, yield strength and the product of ultimate tensile strength and total elongation due to the formation of retained austenite and granular bainite structure. Laminar flow cooling after hot rolling results in a significant increase in the quantity of ferrite and bainite due to the suppression of pearlite transformation, and as a result, the present steel possesses high strengths and good toughness.

Abstract: Precipitation strengthening by fine Nb-rich particles represents an important element on the design of low carbon high strength steels. This is typically obtained on steel strips by thermal holding at temperatures above 600°C following the austenite to ferrite transformation. These conditions are beneficial to obtain a large precipitation of small Nb-rich precipitates. On the other hand, precipitation at lower temperatures, in a phase already hard, such as bainite, has been scarcely studied. In this work, the precipitation phenomena occurring during isothermal treatments following the austenite to bainite transformation at 450°C are investigated. For this purpose, two Nb-alloyed low carbon steels with and without silicon are studied and the evolution of the microstructure is determined by the use of transmission electron microscopy and followed by hardness measurements. The presence of a hardness peak is not detected until long isothermal times (150h). Preliminary atom probe tomography (APT) characterization provides insight on the possible presence of fine NbC precipitates at the peak hardness treatment. A comparison with a Nb-free alloy indicates a significant hardening effect of niobium on the bainitic structure.